At present, shaped bodies of various plastic sheets are used as parts of automobiles, bicycles, motorboats, snowmobiles, domestic electric appliances, electronic devices, and so on. Plastic sheets are also used when coloring, decoration or display is applied to outer casing, interior materials, building materials, various guide plates, traffic signs, indoor/outdoor advertisements, signboards, shutters, windows, and so on or when various surface functions such as weather resistance, stain-proofing property and various kinds of resistances are given thereto. Moreover, various plastic sheets are used as a packaging material for preventing deterioration in quality of various articles or for protecting various functions of various articles.
In many cases, a semihard or soft vinyl chloride resin is used as a material for these plastic sheets. However, in order to give physical properties such as shaping-working properties, extensibility and flexibility to the semihard or soft vinyl resin, a large amount of a low molecular weight compound, such as a phthalic acid plasticizer, is added to the resin. Since the plasticizer has large migrating property, the resin has the following problems: the plasticizer migrates to the surface of a resin product with the passage of time, and damages the external appearance of the product; the plasticizer migrates to a product covered with the sheet and lowers the adhesive force between the product and the sheet; and swelling or creases are generated by expansion of a substrate. Since the plasticizer migrates to the surface of the resin product or the product covered with the sheet, the vinyl chloride resin also has a problem that the extensibility of the resin is lost, so that the state of the resin at the time of shaping processing cannot be kept. Furthermore, the soft vinyl chloride also has a problem that the resin has low hardness and poor abrasion resistance and scratch resistance.
In addition, acrylic resins such as polymethyl methacrylate, polyethylene terephthalate, polyethylene, polypropylene, and the like are generally known as materials for plastic sheets. However, acrylic resins such as polymethyl methacrylate, and polyethylene terephthalate have toughness but have poor extensibility. Therefore, in the case of the sheets made of these resins, the sheets are not easily shaped, in particular, at ordinary temperature. Thus, in order to make the sheets soft, a low molecular weight plasticizer must be added thereto in the same manner, as for vinyl chloride resin. Acrylic resins such as polymethyl methacrylate, polyethylene terephthalate, polyethylene and polypropylene have problems that, when the resins have a high molecular weight, the resins have low capability of dispersing a colorant such as a pigment therein and the resins are not easily colored, so that a decoration-receiving property of the resins markedly lowers. The resins have a problem in that, when the resins have a low molecular weight, the toughness of the sheets made of such a resin is poor.
In addition, as a material for plastic sheets having extensibility, urethane resin and the like are known. However, this has problems of poor toughness, low weather resistance, and the like.
In order to obtain a plastic colored sheet having toughness, extensibility and flexibility, a method of laminating at least two kinds of sheets, which have different physical properties that complement each other, is often used.
Resin compositions having good pigment-dispersibility so as to be easily colored, which have been used in colored layers hitherto, have a low molecular weight in many cases. Thus, if the compositions are used in a monolayer form, the layer has poor toughness. Therefore, it is difficult to use the compositions in the form of the monolayer sheet.
On the other hand, in the case of resins having both toughness and flexibility, such as soft polyolefin or the like, it is difficult to disperse a pigment therein. Thus, the resins are not easily colored and it is substantially impossible to produce colored sheets having various colors and made of such a resin.
For this reason, a method of laminating a colored resin layer having insufficient toughness but having good pigment-dispersibility on a sheet substrate made of a tough, flexible and transparent plastic sheet such as soft polyolefin to produce a tough and flexible colored sheet is used. However, in the case where this method is adopted, the following problems arise: a drop in the adhesion between the substrate layer and the colored layer, in particular, a drop in the adhesion at the time of shaping or extending; shrinkage, warp or peeling at the time of application or lamination or over time, which is due to difference between the thermal shrinkage ratios of the respective layers; and the like. Additionally, costs rise up since the laminating step is also needed.
Soft or semihard vinyl chloride resin has both pigment-dispersibility and toughness, and can be used in a monolayer sheet form. However, the pigment is dispersed in a plasticizer. Therefore, the plasticizer is an essential component and thus it is impossible to avoid various problems which result from use of the plasticizer, as described above.